# Square, cube, octahedron, equations

We know that $|x| + |y| - 1 = 0$ is the equation of a square having its vertices on the axes.

I asked to represent the equation $|x| + |y| + |z| - 1 - 0$, believing to obtain a cube in space.

But I obtain an octahedron. Why? And how do you get a cube?

```
# with SageMath 7.3
var('x, y, z')
f = abs(x) + abs(y) + abs(z) - 1
implicit_plot3d(f, (x, -1, 1), (y, -1, 1), (z, -1, 1), color='aquamarine ')
```